1. Field of the Invention
The present invention relates generally to rotary wing aircraft. In more specific aspects, the present invention relates to the mast that supports the rotary wing system of a rotary wing aircraft and methods for use associated therewith.
2. Description of the Related Art
There are two types of heavier-than-air aircraft that achieve lift by movement through the air that relate to this art: (1) the airplane, and (2) the rotorcraft or rotary wing aircraft. The airplane has stationary wings that create lift when propelled through the air by a thrust mechanism such as a propeller or jet engine. The rotorcraft or rotary wing aircraft has blades that rotate to describe or form a rotor disc (the plane the rotor blades rotate in) positioned above the aircraft fuselage to create lift.
There are three types of rotorcraft that utilize a rotor blade to provide lift: (1) the helicopter, (2) the autogyro, and (3) the gyroplane. In the helicopter, the rotor blades are driven by an engine to rotate to form a plane of rotation which produces a resultant force vector defined as the vector sum of the rotor's lift and drag forces. This rotor disc provides a vertical lift vector or vertical thrust necessary to counteract the weight of the aircraft and thus provides for a vertical velocity. This rotor disc can be tilted on a supporting and/or rotating vertical mast. This tilting of the rotor disc results in a horizontal lift vector or horizontal thrust component, which counteracts drag in order to provide for a horizontal velocity. In the autogyro, vertical thrust is provided by a rotary wing or rotor forming a rotor disc and forward thrust is normally provided by a propeller. Autorotation is achieved by tilting the rotor disc back relative to the airflow so that some air flows up between the blades and through the rotor disc rather than down through the rotor disc as in a helicopter. As the air flows up through the rotor disc, the rotor disc is driven much like a windmill is driven by the wind. In the gyroplane, a rotor forming a rotor disc is used for vertical and slow speed flight, but at high speed cruising the rotor is unloaded (minimal lift) and the wing provides substantially all of the lift.
Trimming moments about the pitch axis of a rotorcraft require the rotor resultant force vector to pass through or near to the aircraft's center of gravity. This resultant force vector is close to and normally assumed to be normal or perpendicular to the rotor disc. This means that for a rotorcraft, such as a helicopter, to move forward against the drag on the aircraft, the rotor disc must be tilted forward (down) to cause the resultant force vector to be oriented forward of vertical. The resultant force vector, however, must remain close to the aircraft center of gravity with the result that the helicopter's fuselage also tilts nose down.
This tendency for the resultant force vector produced by the rotor of a rotorcraft to equalize any moments between the resultant force vector and the aircraft center of gravity is also manifest in other ways. For example, regarding a helicopter in a hover, as a result of the tendency for the resultant force vector to pass through the center of gravity, changing the location of the center gravity from say an aft center of gravity to a forward center of gravity, causes the fuselage of the helicopter to correspondingly tilt from a fuselage nose-up attitude to a fuselage nose-down attitude. Further, the combined effects of forward speed and this tendency for the resultant force vector to pass through the center of gravity, can cause large and sometimes undesirable fuselage attitudes in steady flight. These undesirable fuselage attitudes are made even larger during forward accelerations (often termed “dumping the nose”) and nose-up “flares” typically required for deceleration. These undesirable attitudes are especially prevalent in helicopter designs where the mast of the rotor head has a preset forward tilt implemented to provide the aircraft fuselage with a more level attitude during cruise flight.
Similar to the helicopter, the autogyro also must have its resultant force vector pass through or near the aircraft center of gravity for trim, and hence, the fuselage must also tilt for airspeed changes and center of gravity trim. When the rotor of the autogyro is in autorotative flight, the rotor disc must be titled aft so that the airflow is up through the rotor blade to produce the autorotative force on the rotor blades. This typically means that the fuselage must also be nose-up. The slower the steady forward flight, the more the rotor disc must be tilted aft to generate sufficient vertical lift and hence the higher the nose-up fuselage angle required. Also similar to the helicopter, to slow down (decrease airspeed) or come to a stop, the autogyro typically must flare, nose-up, even further. In a jump take-off, however, the autogyro rotor disc, and thus the fuselage, will be level or nose-down.
Applicant recognizes that the above described fuselage attitudes of both helicopters and autogyros are sometimes uncomfortable to the occupants, and/or can cause loss of pilot visibility, and/or cause difficult and dangerous landing conditions. Adding wings to helicopters to make compound helicopters and adding wings to autogyros to make gyroplanes reduces, to some degree, the otherwise extreme attitudes in high speed flight. With the wing providing the major lift component during forward flight, the rotor resultant force vector is no longer as strong an influence on aircraft pitch trim. Likewise, in high-speed flight, it is not mandatory that the resultant force vector be required to pass close to the aircraft's center of gravity. Similar to a fixed wing airplane, pitch trim can be maintained in high-speed flight with use of a horizontal stabilizer and an elevator.
The winged helicopter and the winged autogyro, however, when in autorotation, can encounter excessive nose-up fuselage attitude in slow speed flight and/or flares, which will stall the wing. This is especially prevalent in extreme aft center of gravity loading conditions. So, like the helicopter and autogyro, the winged rotorcraft can also have design and flying concerns because of extreme fuselage attitudes. Thus, recognized is the need for a means to manipulate the rotor disc such that the resultant force vector is maintained at or near the aircraft center gravity to prevent excessive moments causing excessive fuselage pitching, especially in slow speed flight and during acceleration and deceleration.